Typical known scanning probe microscopes (SPMs) are a scanning tunneling microscope (STM) and an atomic force microscope (AFM). Of these, an AFM comprises a cantilever having a probe at its free end, a displacement sensor for detecting displacement of the cantilever, and a sample stage scanner.
In an AFM, a cantilever is oscillated with a frequency in the vicinity of its resonant frequency by oscillating a piezoelectric element (piezo element), and a probe of this oscillating cantilever is brought into contact with a sample. The sample and the cantilever are relatively scanned in the XY direction and the amplitude of oscillation is detected using a displacement sensor during the XY scanning process. Further, the sample and the cantilever are relatively scanned in Z direction (vertical direction) through feedback process so as to maintain a fixed oscillation amplitude. The variation in height of the sample surface is acquired by the feedback scanning and the shape of the sample can thereby be measured. Thus, in an AFM, the sample shape is measured by detecting with a displacement sensor the variation of oscillation amplitude produced by contact between the sample and the probe.
In an AFM, apart from oscillation amplitude, “phase” is also measured as useful information. It is known that the phase of cantilever oscillation is shifted by the interactions accompanying the energy dissipation of the sample and the probe. The phase information includes physical properties information such as the viscosity, elasticity and composition of the sample. It is therefore possible to follow not merely changes in the structure of living bodies' molecules, but also dynamic changes in physical properties information, by detecting phase change. Phase measurement using an AFM is disclosed in, for example, Miriam Argaman et al., “Phase imaging of moving DNA molecules and DNA molecules replicated in the atomic force microscope”, Nucleic Acids Research, Oxford University Press, 1997, Volume 25, No. 21, pp. 4379-4384.
In a conventional AFM, for example a lock-in amplifier is employed for phase detection. However, a lock-in amplifier does not have sufficient bandwidth such as is required for high-speed AFM measurement. For example, a lock-in amplifier cannot cope with demands for detecting the phase of cantilever oscillation in each individual cycle period. Phase detection technology of higher speed is therefore required.
Also, apart from contact AFMs, non-contact AFMs are known as the AFMs. A non-contact AFM is employed under the condition in which the probe is close to the sample. The problems concerning efficiency of excitation described above are not restricted to contact AFMs. The same problems also occur with non-contact AFMs.